The present invention relates to power supplies for fluorescent lamps, and particularly for emergency operation of fluorescent lamps under battery power in the event of failure of a primary power supply.
While the provision of a battery back-up system for incandescent lamps is a relatively simple matter, emergency operation of fluorescent lamps under battery power poses certain difficulties, including those associated with the special starting requirements of fluorescent lamps.
It is known that a fluorescent lamp can be operated under battery power by supplying the lamp with a high frequency current derived from the battery by an inverter and supplied to the lamp via a ballast capacitor. A switching device is required to switch the circuit from a start mode to an operating mode. In the operating mode, the lamp continues to be supplied with alternating current and, because of the operating characteristics of fluorescent lamps, and particularly their negative resistance characteristic, the power supplied to the lamp during the operating mode cannot be reduced significantly, so that a fluorescent lamp could be operated for only a short period of time under battery power.
An alternative approach to battery powered operation is to convert the converter output into a full wave rectified current which is applied to the lamp. This would permit the lamp, after starting, to be operated at a reduced power level. However, it is difficult to start a fluorescent lamp with rectified current, particularly if the fluorescent lamp has an integral preheat starter.